Scientists have long debated when our ancestors mastered fire, a transformative event that shaped what early people ate and how they lived and may have fueled the evolution of the modern human brain.

Now, the oldest evidence yet for early humans’ use of fire has emerged from a second-floor laboratory at Boston University, where researchers say they have discovered, embedded in ancient reddish slabs of sediment, microscopic flecks of apparent campfires from a million years ago.

The sediment, taken from Wonderwerk Cave in South Africa and stored in modest cardboard file boxes, contained bits of burned bone and plant ashes, a combination and setting that the BU scientists concluded would probably not have been the result of a wildfire.

The find is about 200,000 years older than the previous earliest evidence for human use of fire. But the more lasting significance of the discovery, reported Monday in Proceedings of the National Academy of Sciences, may be that it gives researchers a tool to find other traces of prehistoric humans’ use of fire in archeological sites around the world, a puzzle that has proven difficult to solve.

Unlike durable stone tools, prehistoric evidence of fire is more evanescent and difficult to interpret. Fire is less likely to leave tangible traces, and it occurs naturally, so proving that people were controlling or using fire is challenging.

Estimates of when people began to regularly use fire range from 400,000 to more than 1.6 million years ago. Pinpointing when could help researchers understand when practices like cooking became common, which could have triggered other changes in behavior and social organization, including the division of labor, and enabling activity at night.

The BU team was not looking for evidence of fire. The find was so unexpected that Francesco Berna, a research assistant professor who led the work, found himself trying to poke holes in his provocative observation. But he ruled out that the fires could have been caused by spontaneous combustion of bat droppings or that the signal he was seeing was due to the age of the burned bones, by comparing them with 8-million-year-old bones.

“I had to convince myself and had to convince my colleagues,’’ Berna said. “Scientifically speaking, what is clear is there was fire burning inside the cave of plant material . . . while humans were dropping [leaving behind] tools and bones. It’s not one episode.’’

Along with evidence of fire, manmade stone tools and artifacts were found in the cave.

When people began to control fire is a central question for a number of fields studying the emergence of modern humans.

Harvard University professor Richard Wrangham has put forth the “cooking hypothesis,’’ an argument that the advent of cooked food was crucial in shaping modern humans, because it provided more efficient nutrition to fuel the growth of bigger brains and allowed early humans to spend less time chewing. His theory holds that early humans began cooking with fire more than 1 million years ago.

Wrangham said the finding not only pushes back the date of the earliest known human use of fire, but provides a new tool that may help resolve the question.

“It shows that if you do a more careful analysis, you can get more evidence,’’ Wrangham said. “It’s quite true the further back you go in time the harder it is to find evidence of control of fire; it’s thoroughly understandable because the evidence is going to decay. My guess is that through the imaginative use of all sorts of different methods, including genetic methods, we will indeed find’’ more clues.

The BU scientists started with hunks of the cave floor. Each block of sediment was embedded in a clear polyester resin that could be sawed into smaller, hard chips, mounted on slides, and polished to a thin layer. The scientists could then examine the cave floor in microscopic detail.

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Bone found in a cave in South Africa, which is evidence about when humans first controlled fire.

What Berna found initially was bits of bone that appeared to be burned. Closer examination with his colleague, Paul Goldberg, revealed ash from the cave’s floor. The team, also led by Michael Chazan from the University of Toronto, used archeological techniques to date the samples to 1 million years ago.

Some of the findings are already stirring debate.

“Is the Wonderwerk evidence good enough to suggest that hominins a million years ago were regular fire users all through their range? A definite ‘No,’ we do not have the evidence to back up such a claim, before 400,000 years ago,’’ wrote Wil Roebroeks, an archeologist from Leiden University, and Paola Villa, at the University of Colorado Museum of Natural History.

But John Gowlett, a professor of archeology at the University of Liverpool, said the evidence is an encouraging addition to a pattern emerging as scientists identify older signs of fire use.

Lawrence Straus, an anthropologist at the University of New Mexico, said evidence of ancient fire use may always be cryptic. But because the question is an important one, it will not die out.

“The whole story of our evolution has been one of our, for better or worse, becoming a species that is sort of different in the extent to which we manipulate nature more than other animals,’’ Straus said. “And fire is really a very potent instrument.’’